Tri-color kinescope



TRI-COLOR KINESCOPE 2 Sheets-Sheet 1 Filed June 25. 1953 i add; 4 Am u w xmmfa fi 6 June 26, 1956 MQRRELL 2,752,520

TRI-COLOR KINESCOPE Filed June 25. 1953 2 Sheets-Sheet 2 INVENTOR.

TOR NE 1 United States Patent TRl-COLOR KHNESCOPE Albert M. Morreli, East Petersburg, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application June 25, 1953, Serial No. 364,041 12 Claims. (ill. 313-=-70) This invention is directed to a cathode ray tube and more specifically to a cathode ray tube having a plurality of electron beams which are converged to a common point on a target electrode.

One type of cathode ray tube which uses a plurality of electron beams is the television picture tube for color of the type disclosed in U. S. Patent 2,595,548, Alfred C. Schroeder. It has been found advantageous in tubes of this type to utilize an electron gun means which has three electron guns mounted in the neck of the tube envelope symmetrically about the tube axis to provide three substantially parallel beams directed at a target electrode mounted normal to the beam paths. Either an electrostatic or a magnetic electron lens may be used to converge the parallel beams to a common point at the target electrode and in a manner that each beam approaches the target at a small angle of incidence and from a different direction. The target electrode of tubes of this type may consist of a masking apertured electrode formed of sheet metal for example copper-nickel alloy and having a large number or" small apertures therethrough. Closely spaced from the surface of the masking electrode opposite the electron guns is a glass sheet having, on its surface adjacent the masking electrode phosphor dots in groups of three, with each dot of each group fluorescing with a difierent color of light than the other dots of the group when struck with high energy electrons. Each group of three dots is positioned relative to one aperture of the masking electrode so that the center about which the three dots of each group is positioned is aligned with the respective aperture of the mesh. Electrons from the three beams, passing through any one of the apertures in the masking electrode, approach the corresponding group of phosphor dots from their three different directions. In this manner, the electrons passing through an aperture of the masking electrode along one direction will strike only one phosphor dot and will be prevented by the masking electrode from striking the other two dots of the group. The arrangement provides, then, that an electron beam from each gun of the tube will cause the phosphor screen to luminesce in only one color.

Tubes of the type described have successfully used an electrostatic lens field for converging the electron beams to a common point at the masking electrode. The electrostatic lens is formed between two common accelerating electrodes through which the beams pass. The difference in potential between the electrodes establishes a common converging field having an axis coinciding substantially with the tube axis. However, the use of such a converging lens field requires that the three electron guns be quite accurately disposed symmetrically about the axis of the lens field so that the three beams will pass through identical portions of the lens field to provide convergence of the three beams to a single point.

Accurate convergence of the three beams also depends upon precise alignment of each gun and the gun parts.

Misalignment of the parts of a gun results in the displacement of the beam from its path of optimum beam convergence. A single electrostatic converging lens field, of the type described, has no separate control of each beam so as to correct for misalignment of gun parts which may inherently occur in tubes of the type described. To correct for the effect of misalignment of the beams and to provide correct convergence of the beams at the target additional services are required. These devices, such as individually mounted permanent magnets mounted on the tube neck, usually caused a distortion of the beam and hence reduced resolution.

It is, therefore, an object of this invention to provide a cathode ray tube having a plurality of electron beams which can be easily converged to any point on the target, with no adverse effect on focus.

It is another object of the invention to provide a color television tube utilizing a plurality of electron beams which can be independently converged to any point on the target of the tube.

it is another object of the invention to provide a color television tube having a plurality of electron beams which can be independently controlled to provide convergence of the beams at the target of the tube.

The invention makes use of magnetic converging means for bringing the three beams of a color tube to any point on the target electrode. Independent convergence control is provided by a plurality of pairs of pole pieces. Portions of the pole pieces are positioned adjacent to the envelope neck of the tube whereby an electromagnet mounted on the tube neck adjacent each pair of pole pieces will establish a magnetic field across each beam path to direct the electron beam toward the axis of the tube. By adjusting the magnetic field between each pair of pole pieces, the three electron beams can be brought substantially to a point anywhere on the target of the tube. An additional pair of deflecting plates is mounted in one of the three guns to provide a correcting magnetic field at right angles to the other field so that convergence of the three beams can be obtained in spite of any misalignment of gun parts which produce misconvergence of the beams at the target.

Figure 1 is a sectional view of a utilizing the invention.

Figure 2 is an enlarged perspective view of a portion of the target of the tube in Figure 1.

Figure 3 is an enlarged sectional view of part of the gun structure of the tube of Figure 1.

Figure 4 is a cross sectional view along line 4-4 of Figure 3.

Figure 5 is a cross sectional view along line 5-5 of Figure 3.

Figure 6 is a partial sectional view of a modification of the'invention shown in Figure 3.

Figure 7 is a cross sectional view along line 77 of Figure 6.

Figure 8 is a schematic representation of beam convergence at the target electrode of the tube of Figures Figure l discloses a cathode ray tube having a plurality of electron beams and used as a picture viewing tube for color television. The tube consists of an evacuated envelope having a neck portion 1%, for example of glass, and a shell portion 12 which may be either frusto-conical or substantially the frustum of a pyramid, as is well known in the art and which may be of metal. Within the neck portion 10 of the envelope are three electron guns 13 shown in greater detail in Figure 3. The guns 13 each consist of a cathode electrode 14 mounted within a tubular control grid electrode 16. Each control grid 16 is closed at one end by a wall 15 having a single aperture at its center. The adjacent end of cathode ray tube each cathode electrode 14 is closedby a solid wall portion and is coated on its outer surface with electron emitting material, such as a mixture of barium and strontium oxides, to provide a source of electron emis- SlOn.

Closely spaced from the control grid 16 of each electron gun' and along a common axis, there is' mounted a short tubular or cup-like accelerating electrode 20 having an aperture in the bottom of the cup in line with the aperture in the control grid wall 15. Spaced along the axis of each gun from accelerating electrode 20 is a relatively long tubular accelerating and focusing electrode 22, closed at its end facing accelerating electrode 20 by an apertured wall portion 23, whose aperture is also aligned with the apertures in electrodesZti and 16. Closely spaced from the other ends of electrodes 22 is a common electrode 24, consistingof a tubular member closed by an apertured walhportion 26 at the far end and having mounted in the opposite wall portion 28 a plurality of short tubular members 30. One of the tubular members 30 is aligned on the common axis of each electron gun with the other electrodes 16-2!J and 22. Within the wall 26 of electrode 24 there is formed an aperture 32 on the axis of each gun and also in alignment with the apertures of electrodes 16, 20 and 22. Electrode 24 is supported from and electrically connected by spring fingers or spacers 34 to the wall of the envelope neck 10. The spring fingers make electrical and physical contact with a conductive wall coating 36, which extends over the inner surface of the tubular envelope neck portion from electrode 24 into the conical envelope portion 12 to make contact with the metal shell 12.

The several electrodes are mounted within the neck portion 10 by rigidly fastening them together by means of glass mounting rods 38. Each electrode has one or more studs 40 welded at one end to the electrodes and sealed at the other end into a glass rod 38 in a conventional manner. Lead wires 42 extend from each electrode to supporting base wires (not shown) sealed through the base 43 of the tubular envelope neck portion 10 in a conventional manner. By lead wires 42 and the spring spacing fingers 34, the electron guns are rigidly supported within the envelope neck portion It).

In figure 3, voltages are indicated as those which are applied to the respective gun electrodes. These voltages are those which have been successfully used in tubes of the type described and need not be limiting.

During the operation of the several electron guns, potentials are applied to several gun electrodes in the amounts indicated. The electron emission from each cathode 14 is formed by electrostatic fields respectively between electrodes 16, 2t), and 22 into an electron beam directed through the apertured portions of the gun electrodes. The difference of potential between electrodes 22 and the corresponding electrode portions 34) provide a principal focusing lens field in the path of each electron beam, whereby the electrons of each beam are converged to a fine focus at the target electrode 44 v mounted in the large portion of the conical envelope 12.

Target 44 consists of a glass support plate 46 and a metallic masking electrode 48 closely spaced from the surface of plate 46 facing the electron guns. Masking electrode 48 is of a thin copper-nickel sheet having a large number of small apertures 50. Fixed to the adjacent surface of the glass plate 46 are groups of phosphor dots 52, with each group consisting of three dots positioned in a triangular arrangement about a center point 54, as shown in Figure 2. The positioning of each group of phosphor dots is such that the center of each aperture 50, in the masking electrode 48, will be aligned with a point 54 of the corresponding group of phosphor dots. Figure 2 shows schematically a line N from point 54 through the respective aperture 59.

The phosphor dots 52 of each group are'formed of phosphor material fluorescing with a different colored 'pole pieces.

4 light when struck by the high energy electrons from guns 13. As indicated in Figure 2 the dots of each group have a red, green, or blue fluorescence under electron bombardment and indicated respectively by R, G, and B. Furthermore, the positioning of the phosphor dots 52 is that in which each dot is aligned with its corresponding aperture 50 in electrode 48 along a different directional line X, Y, and Z, respectively.

The three electron beams leaving the electron guns 13 are caused to converge toward the common axis of the tubular envelope portion 10. The converging means, described in greater detail below, comprises magnetic fields in the paths of electron beams which direct each beam toward axis 17 to converge the three beams to a point on the masking electrode 43. Each beam approaches the masking electrode 48 at a small angle of incidence and from one of the different directions X, Y, or Z. Electrons from each beam passing through the apertures 50 of electrode 48 along paths extending in the directions X, Y, or Z will strike one of the phosphor dots in each group of dots aligned with each aperture 50. The arrangement is such then that the electrons from each gun can strike only those phosphor dots 52 luminescing with a single color of light.

The three beams are simultaneously scanned over the surface of the masking electrode 48 by conventional scanning means indicated as a neck yoke 56, which consists of two pairs of deflecting coils, with the coils of each pair mounted on opposite sides of the envelope neck 10. Each pair of deflecting coils of yoke 56 is connected in series to sources of saw tooth currents for providing line and frame scansion of the three electron beams simultaneously over the surface of the masking electrode 48. The scanning coils of yoke 56 are conventional and do not constitute a part of this invention and need not be further described. The scansion of electron beams may be in any desired manner but for color television viewing is as a rectangular raster.

During tube operation, the three electron beams of the tubes of Figures 1 and 3 are converged in addition to the focusing action of the gun by magnetic means to any point on the target 44 as described above. in accordance with the invention, the magnetic converging means consists of pairs of pole pieces 57 mounted with one of the beam 'paths passing between each pair of As shown specifically in Figures 3 and 4, the pole pieces include parallel plate portions 58 extending substantially radially to the axis 17 of the envelope neck 10. The pole piece portions 58 pass through and are held by the wall of the tubular electrode 24 and are adjacent to the beam focusing extensions 36. Furthermore, pole pieces 57 have arcuate armature portions 60 extending along the inner wall portions of the tubular neck 10, as shown in Figure 4. The arcuate armature portions 6% are matched with arcuate portions 62 of armatures 64 forming a part of an electromagnet 66 mounted on the outer wall portion of the tubular neck 10 for energizing the pole pieces 58.

The parallel plate portions 53 of each pair of pole pieces 57 extend on opposite sides of each electron beam path. The operation is such that each electron 'beam will pass between a pair of pole portions 58 during tube operation. Magnetic coils 66 may comprise either single or double coils or a combination of permanent and electromagnets for providing static and dynamic convergence respectively of the electron beams to any point on the target 44. The passage of current through one of the coils in the magnet coils 66 will cause deflection of. the corresponding electron beam in a direction parallel to the plate portions 58. With correct polarity, the field established between each pair of pole pieces 58 will direct the .corresponding electron beam radially toward the axis 17 of the tube. Instead of coils 66, perrnanent magnetic means may be used with anadjustment which will permit varying the distance of the pole pieces of the permanent magnets from the plate portions 60, so that the amount of static convergence for each beam can be controlled. By adjusting the strength of the mag netic field between each pair of plates, it may be possible to converge the three electron beams to substantially the same point at the target.

However, due to misalignment of the gun parts as well as nonuniformity of the focusing fields, the three beams may not normally coincide accurately at the same point. Figure 8 indicates that if the three beams are accurately positioned symmetrically about axis 17, and with no deflection fields acting on the beams, they will strike three spots R, B and G on target 44. The three spots R, B and G will be spaced 120 apart about the center of axis 17. By establishing the correct strength of field between each pair of plates 58, the three beams spots R, B and G are moved toward the tube axis 17 along radial paths spaced apart by angles as determined by the orientation of the pole pieces, in this case 120 as indicated by the arrows 61, 63 and 65, respectively, shown in Figure 8 and the three beams can be brought to a common point of convergence on the axis 17. However, due to misalignment of gun parts or nonuniformity of fields through which the beams pass or both, one or more of the spots R, B and G may be displaced from the spots which they should theoretically strike and will strike in new spots R and B, for example. The action of the fields between corresponding plates 58 now is to move the three spots R, B and G together along the lines 61, 63' and 65 respectively, and as indicated by the arrows as shown in Figure 8. However, the three beams can not now be converged to a single common point, although any two beams can always be brought to a point of convergence. Accordingly then, a correction is introduced into the deflection of one of the beams by parallel pole pieces or plates 70 as shown in Figures 3 and 5, for example. Pole pieces 70 are fixed within the tubular electrode 22 of the blue gun 13 and are mounted at substantially right angles to the corresponding plates 58 of the gun. A magnetic field between the parallel pole pieces 79 will introduce a beam deflection at right angles to that produced by plates 57 on the same beam. As shown in Figure 8, this produces resultant deflection of the beam B along the path of arrow 63", so as to shift the position of B to the intersection of spots G and R and to provide a common convergence of all of the beams at the point 0'. The point 0' is not greatly displaced from the theoretical 0. However, this displacement can be compensated for by other means during tube operation. However, it is within the scope of this invention to use correcting plates similar to plates 70 for each beam respectively to avoid any displacement.

The correcting magnetic field established between pole pieces 70 may be formed by extending pole pieces 70 adjacent to the neck portion of the tube envelope to form arcuate armature portions 72. Arcuate armature portions 74 of an armature 76 are positioned on the outer surface of the tubular neck portion overlying the arcuate pole piece portions 72. The polarity of the armature portion 74 can be established by a magnetic coil winding '78 around the central portion of armature 76. This arrangement thcn, establishes the magnetic correcting ield between the plates 70. The strength of this field can be varied by the amount of current passed through the coil. 78, or a permanent magnet with a variable shunt or other controlling device may be used.

As the three beams are simultaneously scanned over the surface of target 44, as described, the beams will lose convergence at the target as the distance of beam convergence from masking screen 48 varies due to the geometry effects connected with the deflection of several converging beams. It is thus necessary to provide a dynamic magnetic field in the path of each beam. This may be done by a second coil in each of the magnets 66 to provide the dynamic or changing converging 6 field between each pair of plates 57, to maintain beam convergence at the mask 48, as the beams are deflected thereover. This is not part of the present invention.

One of the main advantages of the present invention is that there is now provided a stable operation of the convergence of the three beams at the target mask 48 at all times. One of the disadvantages of tubes which depend upon an electrostatic converging field is the instability of beam convergence due to variations in the voltages applied to the field forming electrodes. Fluctuations of power supply, leakage bet-ween electrode elements and electrode leads, or other conductors, for example, cause the voltages of the electrodes to fluctuate during tube operation. If such voltages are changed relatively only a small amount, the convergence of the three beams at the masking electrode 48 is varied considerably. Use of magnetic fields for beam convergence in accordance with the invention and as described, thus permits the elimination of voltage regulators and such devices from the power supply of the tube as they are not now needed.

The invention utilizes separate magnetic converging fields, one for each beam and in no way depends upon beam convergence through the use of voltages applied to the tube electrodes. Thus, normal fluctuations of electrode voltages, as set forth above, in no way affect the beam convergence of the tube described above for this invention.

A further advantage of the invention is that different dynamic voltages can be applied to each gun as is required by the geometry of the tube set forth above. Because of the complexity of using three beams, the convergence of any pair of beams is difierent than the convergence of any other pair of the three beams during scansion of all three beams across target 44. Because of this then, it is helpful to apply different dynamic voltages to each of the three coils 66 to provide common convergence of all three beams at the target 44 during the complete scanning of the rectangular raster. Such a result would be unobtainable to the same degree with common electrostatic convergence of the three beams, as only a single dynamic voltage could be applied between the electrodes providing the common converging electrostatic field.

The separate control of the three beams by the individual magnetic fields between pairs of plates 57 and provides means for overcoming any inherent problems of convergence related to each gun or the three guns totions 60, 62 and 64 are made or" material of high magnetic permeability, while the supporting members and gether.

The pole pieces 58 and 70 as well as the armature porother parts of guns 13 are made of non-magnetic material.

Figures 6 and 7 show an alternative form of electrode 24 of Figures 1, 2, and 3. The tubular focus portions 30 of the electrode are mounted in a plate portion while the apertured plate 26 of the electrode closes a short tubular section 32. The pole pieces 58 for the three beams are fixed between plate 88 and a second plate 84 closing the adjacent end of the tubular portion 82.

The apertured plate 26 and 26' of electrodes 24 and 82, respectively, Figures 3 and 6, may be made in the form of a magnetic shield. For example, as shown in Figure 6, the plate 26' is formed of iron-nickel plates 88 and a copper plate 96'. The laminated wall 26 thus provides a magnetic shield structure for shielding the magnetic converging fields between plates 58 from the deflecting fields of the yoke 56.

While certain specific embodiments have been illus trated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. An electron discharge device comprising, electron gun means "including a source of 'apluralityof beams along respective paths in 'a common electrons for providing general direction, a target electrode mounted transversely to said beam paths, and means between said electron source and said target for providing a magnetic field in the path of each electron beam for converging said beams at said target, said beam converging 'means including a plurality of pairs of pole pieces mounted with one of said beam paths passing between each pair of said pole pieces, and other means mounted on said gun means and associated with only one of said beams for providing a second and separate magnetic field transverse to said one beam path and to the other transverse field associated with said one beam. Y

' 2; 'An electron discharge device comprising an envelope having a tubular neck portion, electron gun means including a source of electrons for providing a plurality of'bearns' along respective paths in a common general direction, a target electrode mounted in said envelope transversely to said beam paths, and means within said envelope between said electron source and said target for providing a magnetic field in the path of each electron beam for converging said beams at said target, said beam converging means including a plurality of pairs of elongated parallel pole pieces extending substantially radially from the axis of said neck portion with one of said beam paths passing between each of said pairs of pole pieces, said pole pieces each having arcuate portions positioned adjacent the inner wall of said neck portion to be magnetically coupled to an external magnet.

3. An electron discharge device comprising, electron gun means including a source of electrons for providing a plurality of beams along respective paths in a common general direction, a target electrode mounted transversely to said beam paths, and means between said electron source and said target for providing a magnetic field in the path of each electron beam for converging said beams at said target, said beam converging means including a plurality of pairs of pole pieces, each of said pairs of pole pieces including parallel plate portions extending on opposite sides of respective beam path, said electron gun means including magnetic pole pieces for correcting misconvergence of said beams.

4. An electron gun means comprising, a source of electrons for providing a plurality of beams along respective paths in a common general direction, a focusing electrode having apertured portions for the passage therethrough of said electron beams, a plurality of pairs of pole pieces fixed to said focusing electrode, said pole pieces mounted with one of said beam paths passing between each pair of pole pieces thereof.

5. An electron gun means comprising a source of electrons for providing a plurality of beams along paths symmetrically disposed about a common axis, a tubular focusing electrode coaxially mounted on said axis and enclosing said beam paths, and means for providing a mag netic field in the path of each electron beam for converging said beams to a common point, said beam converging means including a plurality of pairs of parallel plates fixed to said focusing electrode, each of said pair of plates radially positioned about said axis with one of said beam paths passing therebetween.

- 6. A cathode ray tube comprising, an envelope having .a tubular neck portion, a plurality of electron guns within said envelope neck portion each including a source of electrons and a plurality of electrodes mounted in succession along an axis, means symmetrically mounting said guns about a common axis to provide a plurality of elecq tron beams along paths symmetrically spaced from said common axis, a target electrode mounted within said envelope transversely to said common axis, a tubular electrode coaxially mounted on said axis and enclosing sai'd beam paths, said tubular electrode having a pluralityof beam focusing portions with one of said focusing 'portions included as a part of each of said electron guns,

and means for providing a magnetic field in the path of each electron beam for converging said beams mammmon point on said target, said beamconverging means including a plurality of pairs of pole pieces mounted on said tubular electrode, each of said pair of pole pieces consisting of parallel plate portions extending with one of said beam paths passing therebetween, and each of said pole pieces including an arcuate portion positioned adjacent the inner wall ofsaid tubular neck envelope portion.

7. A cathode ray tube comprising, an envelope having a tubular neck portion, a plurality of electron guns each including a source of' electrons and a plurality of electrodes mounted'in succession along an axis, means symmetrically mounting said guns within'said envelope neck wortion about. a common'axis to provide a plurality of electron beams along paths symmetrically spaced from said comon axis, a target electrode mounted Within said envelope transversely to'said common-axis, a tubular electrode coaxially mounted on said common axis within said envelope and having a plurality of beam focusing portions with one of said focusing portions included as a part of each of said electron guns, and means for providing a magnetic field in the pathof each electron beam for converging said beams on said target, said beam converging means including a plurality of pairs of pole pieces fixed to said tubular electrode, each of said pair of pole pieces consisting of parallel plate portions extending substantially radially from said common axis with one of said beam paths passing therebetween, and arcuate plate portions integral with each one of said parallel plate portions and coaxially positioned to said common axis adjacent said tubular neck envelope portion, and one of said electron guns including means for correcting misconvergence of said beams.

8. An electron discharge device comprising, an envelope having a tubular neck portion, electron gun means mounted within said envelope and including a source of electrons for providing a plurality of beams along respective paths in a common general direction, a target electrode mounted in said envelope transversely to said beam paths, and means between said electron source and said target for providing a magnetic field in the path of each electron beam for converging said beams at said target, said beam converging means including a plurality of pairs of elongated parallel pole pieces extending substantially radially from the axis of said neck portion, with one of said beam paths passing between each of said pairs of pole pieces, said pole pieces each having arcuate portions positioned adjacent the inner wall of said neck portion to be magnetically coupled to an external magnet, and means mounted on said tubular neck portion adjacent said arcuate pole pieces for energizing said beam converging means.

9. An electron discharge device comprising, an envelope having a tubular neck portion, a plurality of electron guns within said envelope and each including a source of electrons and a plurality of electrodes mounted in succession along an axis, means symmetrically mounting said guns about a common axis to provide a plurality of electron beams along paths symmetrically spaced from said common axis, a target electrode mounted within said envelope transversely to said common axis, a tubular electrode coaxially mounted in said envelope on said axis and having a plurality of beam focusing portions with one of said focusing portions included as a part of each of said electron guns, means for providing a magnetic field in the path of each electron beam for converging said beams at said target, said beam converging means including a plurality of pairs of parallel plates fixed to said tubular electrode with one of said beam paths passing between each pair of said parallel plates, one of said electron gu'ns including parallel pole pieces spaced substantially radially from said common axis on opposite sides of one of said beam paths, and a plurality of mag netic coils mounted on said tubular neck portion for energizing said beam converging means.

10. An electron discharge device comprising, electron gun means including a source of electrons for providing a plurality of beams along respective paths in a common general direction, a target electrode mounted transversely to said beam paths, and means between said electron source and said target for providing a magnetic field in the path of each electron beam for providing convergence of said beams at said target, said beam converging means including a plurality of pairs of magnetic pole pieces mounted with a different one of said beam paths passing between each pair of said pole pieces.

11. An electron gun means comprising, a source of electrons for providing a plurality of beams along respective paths in a common general direction, and means including a plurality of pairs of magnetic pole pieces mounted with a different one of said beam paths passing between each pair of said pole pieces for providing convergence of said beams to a common point, said beam converging means including means for correcting misconvergence of said beams.

12. An electron gun means comprising, a source of electrons for providing a plurality of beams along respective paths in a common general direction, and means for providing a magnetic field in the path of each electron beam for providing convergence of said beams to a common point, said beam converging means including a different pair of magnetic pole pieces mounted on opposite sides of each one of said beam paths, said electron gun means including a second pair of pole pieces mounted on opposite sides of one of said beam paths for correcting misconvcrgence of said beams.

References Cited in the file of this patent UNITED STATES PATENTS 2,211,614 Bowie Aug. 13, 1940 2,220,303 Tingley Nov, 5, 1940 2,457,175 Parker Dec. 28, 1948 2,539,156 Ostreicher Jan. 23, 1951 2,591,159 Kabuss Apr. 1, 1952 2,707,248 Goodrich Apr. 26, 1955 

